This invention relates to general microwave testing and in particular to testing of differential microwave transistors (DUT) in the frequency and time domain using Load Pull (see ref. 1). Load pull is the method by which the load impedance presented to the DUT at a given frequency is changed systematically and the DUT performance is registered, with the objective to find an optimum depending on the overall design objectives. This may be maximum power, efficiency, linearity or else.
Each port on a connectorized RF device comprises two terminals. When one terminal connection is used to transmit the RF signal and the other is used as a ground reference, the port is referred to as “single-ended”. Traditionally, most RF devices have been designed to operate in this mode. When a terminal is designed to reference a signal on another terminal (and not the ground terminal), it is operating in a “differential” mode (see ref. 3). The terminal pair is known as a differential or “balanced” port. These circuits are designed to have a pair of electrically symmetrical signal paths. Signals are transmitted through the device 180 degrees out-of-phase with respect to one another. Any signal that is “common” or in-phase to both terminals will ideally be rejected, and will not pass through the circuit. This characteristic gives the device a lower sensitivity to electromagnetic interference (EMI).
A differential load pull setup is shown in FIG. 2. A single ended signal source injects power into a BALUN (see ref. 4). A BALUN (Balanced-Unbalanced) is a component which generates a differential signal from a single ended one or the opposite. It has three terminals; the input terminal relative to a common (ground) terminal creates a single ended input/output port and two terminals create the differential output/input port. BALUNs can be used in both directions. The accuracy of the measurement depends on the precision with which the phase opposition and the amplitude equality of the signals at the differential port. Ideally the two signals must have the same amplitude and a phase difference of 180°. In reality this is never the case. Therefore an efficient test system shall provide for continuous and fine adjustment of the differential signal components, both in amplitude and phase, both before and after the DUT. In FIG. 2 this is done using the Variable Attenuation and Phase Shifter controllers (VAPS, 21, 22). The differential tuners (23, 24) create true differential impedance presented to the differential DUT (25) at both its input (26) and output (27) terminals. Since the VAPS has a minimum attenuation this must be compensated by fixed attenuators (28, 29). For this and a number of other test applications this invention discloses automated and programmable wideband attenuation and phase controllers.